I don't mind having new characters for the online spin off title, as it is not just Boston that has to deal with the Infected running amok. But then people who are huge fans of the first game will complain about Ellie not being the star anymore.

When a series gets established, people seem to act as if there's just supposed to be those main characters. Anybody else is unfairly branded as being inferior in their eyes. But it's a big universe. So there's no reason not to have other characters introduced, so long as they're not the tedious type.